Effects of autonomic blockade on the inotropic and electrophysiologic response to digitoxin in the intact dog.

Inotropic and electrophysiologic effects in dogs of a single dose of digitoxin with and without autonomic blockade with propranolol plus atropine were observed for 8 hr. Left heart catheterization and His bundle registration were performed in 14 Labrador dogs anesthetized with pentobarbital. Pacing and programmed electrical stimulation were used to determine heart rate-independent changes in dP/dt, intra-atrial, AV nodal, His-Purkinje, and intraventricular conduction velocity. The effective and the functional refractory periods of the atrium, and the functional nodal refractory period (AV-FRP) were measured. Six dogs were given propranolol 0.5 mg/kg plus atropine 0.05 mg/kg every hour. After determination of inotropic and electrophysiologic parameters, digitoxin (2.0 mg) was given to 5 of the dogs, while 1 dog served as control. The other 8 dogs were given only digitoxin (2.0 mg). The dP/dt increased after digitoxin in both groups with maximum values after 2 hr. The autonomic nervous system had only minor influence on changes in contractility caused by digitoxin. The marked fall in heart rate and increase in A-H time (i.e., AV nodal conduction time) and AV-FRP after digitoxin were almost completely blocked by atropine and propranolol. Minor increases in A-H time and AV-FRP indicated a slight direct effect of digitoxin on the AV node.